I think it's decently sharp at 300mm when stopped down. Below 300mm it's sharp at 5.6. At least that has been my experience. You really need to stop down at 300mm to at least F7.1 - F8. Below 300mm, let's say 270mm, you can get pretty sharp shots at 5.6, but at 300mm the lens needs to be stopped down for best performance. I have found the lens to be very sharp wide open at 70mm.

This is at 300mm 5.6 - came out pretty sharp. Center focus point and recompose. No tripod used and no PP on the sharpness, although I have in the camera sharpness bumped up one notch.

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This is stopped down a bit and handheld.

As you can see - I really like this lens. I think it's a keeper. Keep practicing with the lens and you'll better and better with the sharpness. You just need to get to know the nuances of the lens.